Dates and rates of endo-exorheic drainage development: Insights from fluvial terraces (Duero River, Iberian Peninsula)

Abstract

Fluvial terraces are valuable records to study and characterize landscape evolution and river response to base level lowering, and to decipher coupled responses between fluvial incision and regional tectonics. The opening of closed basins has a strong impact on fluvial dynamics, as it involves an abrupt base level lowering that accelerates landscape fluvial dissection. This study focuses on the time response of the Duero Basin, the largest and best preserved among the Cenozoic basins of the Iberian Peninsula, to exorheism. Fluvial incision due to basin opening has developed up to 13 un-paired strath terraces along the south margin of the Duero river, distributed at relative heights up to +136–128 m compared to the modern floodplain. Paired 10Be26Al cosmogenic isotope depth profiles from six fluvial terraces, located ca. 30–80 km upstream from the opening zone, suggest Pleistocene ages for almost the entire fluvial terrace staircase (from T3 at +112 –107 m, to T12 at +13–11 m). The terrace density and the total lowering of the terrace surface, key parameters in limiting terrace exposure ages, were estimated based on field and geomorphological data. Apparent burial durations and basin denudation rates deduced from inherited 10Be26Al concentrations provide valuable information on basin evolution. Apparent basin denudation rates remained relatively low (<3–6 m·Ma−1) during the Pliocene, and doubled (8–13 m·Ma−1) during the Early Pleistocene (ca. 2–1 Ma) possibly showing a lower proportion of recycled sediments. Time averaged incision rates deduced from terraces in the study area and along some tributaries show that incision rates are higher close to the opening site (122–<250 m·Ma−1) than towards the upstream part of the catchment (88–68 m·Ma−1), evidencing the retrogressive travel of the erosive wave nucleated at the opening site.